SQL is a standard query language for efficiently managing data in a database. In a SQL interview, candidates are asked questions about writing different types of SQL queries, understanding database structures, and optimizing database performance.
In this introduction, we will discuss the most frequently asked question types for SQL interviews, such as Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL), aggregate functions, joins, subqueries, etc. We will also provide some examples for each question type.
Top SQL Interview Questions and Answers
1. What is SQL?
Answer: SQL (Structured Query Language) is a standard language used to manage databases. It is used to retrieve, insert, update and delete data.
2. What are the different types of SQL?
Answer:
DDL (Data Definition Language): Defines the structure of the database (CREATE, ALTER, DROP)
DML (Data Manipulation Language): Manipulates data (SELECT, INSERT, UPDATE, DELETE)
DCL (Data Control Language): Controls rights for database users (GRANT, REVOKE)
3. What is a SELECT query?
Answer: SELECT query is used to retrieve data from the database.
4. What is WHERE clause?
Answer: WHERE clause is used to select only those records from the table that satisfy the given conditions.
5. What is FROM clause?
Answer: FROM clause specifies from which table the data is to be taken.
6. What is ORDER BY clause?
Answer: ORDER BY clause is used to sort the data based on a particular column (ASC or DESC).
7. What is GROUP BY clause?
Answer: GROUP BY clause is used to group the records having same values into groups.
8. What is HAVING clause?
Answer: HAVING clause is used to impose conditions on grouped data.
9. What is JOIN?
Answer: JOIN is used to combine data from two or more tables.
10. What is INNER JOIN?
Answer: INNER JOIN returns only those records which are present in both the tables.
11. What is LEFT JOIN?
Answer: LEFT JOIN returns all records of the left table and the records of the right table that match them.
12. What is RIGHT JOIN?
Answer: RIGHT JOIN returns all records of the right table and the records of the left table that match them.
13. What is FULL OUTER JOIN?
Answer: FULL OUTER JOIN returns all records of both the tables, whether they exist in the other table or not.
14. What is UNION?
Answer: UNION combines the results of two or more SELECT queries.
15. What is UNION ALL?
Answer: UNION ALL also combines the results of two or more SELECT queries, but excludes duplicate records.
16. What is SUBQUERY?
Answer: SUBQUERY is a SELECT query that is nested inside another SELECT query.
17. What is EXISTS operator?
Answer: EXISTS operator returns TRUE when the subquery returns any records.
18. What is IN operator?
Answer: IN operator returns TRUE when the value of the column exists in the list.
19. What is BETWEEN operator?
Answer: BETWEEN operator is used to select values within a range.
20. What is LIKE operator?
Answer: LIKE operator is used for pattern matching.
21. What is DISTINCT?
Answer: DISTINCT is used to remove duplicate records.
22. What is COUNT(*)?
Answer: COUNT(*) returns the total number of records in the table.
23. What is SUM()?
Answer: SUM() returns the sum of numeric values in a column.
24. What is AVG()?
Answer: AVG() returns the average of numeric values in a column.
25. What is MIN()?
Answer: MIN() returns the minimum value in a column.
26. What is MAX()?
Answer: MAX() returns the maximum value in a column.
27. What is INSERT INTO?
Answer: INSERT INTO is used to add new records to a table.
28. What is UPDATE?
Answer: UPDATE is used to modify existing records.
29. What is DELETE?
Answer: DELETE is used to delete records from a table.
30. What is TRUNCATE?
Answer: TRUNCATE deletes all the data from the table but maintains the structure of the table.
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Conclusion:
In SQL interviews, questions cover various topics such as database design, query writing, performance tuning, and transaction handling. Candidates are expected to understand the basics of SQL, be able to write complex queries, design database structures, and optimize database performance. The ultimate goal of the interview is to determine whether the candidate has the required skills and knowledge in database management.
